Multimetallic catalysts, i.e. catalysts comprising two or more different metals on a substrate, have been found to be useful in many different processes. Among these are the reforming of petroleum naphthas to high-octane gasoline, isomerization, hydrocracking, desulfurization and hydrogenation.
Multimetallic catalysts are generally prepared by impregnating a high surface area carrier or substrate with an aqueous solution containing appropriate concentrations of the desired metals. A plurality of metals may be present within a single impregnating solution. Alternatively, a substrate may be contacted with a plurality of impregnating solutions, each containing only one metal component.
Conventional impregnation procedures have been found to suffer from an inability to evenly disperse metals throughout the catalyst surface. Consequently, it often becomes necessary to use excessive amounts of metal in the impregnating solution in order to produce a catalyst of optimal activity. Molybdenum-tungsten dehydrogenation catalysts may require metal loadings greater than 9 wt. percent molybdenum and 10 wt. percent tungsten when produced conventionally. Such high metal requirements often make the use of such catalysts economically unattractive.
Alternatives to conventional impregnation methods are known in the art. These methods comprise contacting a non-aqueous solution containing a metal pi-complex with a substrate having a catalytic surface capable of effecting deposition of said metal or metals from the complex thereon. The metal pi-complex can be deposited on the substrate by reducing the complex in the presence of the substrate by means of a reducing agent such as hydrogen gas. Such a method is described in U.S. Pat. No. 3,635,761 to Haag et al., incorporated herein by reference. The art also teaches a method of anchoring a molybdenum metal pi-complex to the surface hydroxyls present in a mixture of silica and alumina. The resulting catalyst material can then either be sulfided by the treatment with H.sub.2 S or, reduced by treatment with hydrogen and then sulfided, or else oxidized and then sulfided. The treated catalyst contains about 2% by weight molybdenum and is found to be a more effective hydrodesulfurization catalyst than its conventionally impregnated counterpart, owing to a more even dispersal of metal, (see, Yermakov et al, Thiophene Hydrogenolysis on Supported Molybdenum Catalysts Prepared Through Mo(.pi.--C.sub.3 H.sub.5).sub.4, React. Kinet. Catal. Lett., Vol. 14, No. 2, 155-160).